Enhancements in automotive safety systems over the past several decades have provided dramatic improvements in vehicle occupant protection. Presently available motor vehicles include an array of such systems, including inflatable restraint systems for the protection of occupants from frontal impacts, side impacts, and roll-over conditions. Advancements in belt restraints and vehicle interior energy absorbing systems have also contributed to enhancements in safety. Many of these systems must be deployed or actuated in a non-reversible manner upon the detection of a vehicle impact or rollover event to provide their beneficial effect. Many designs for such sensors are presently used to detect the presence of an impact or roll-over condition as it occurs.
Attention has been directed recently to providing pre-crash triggered deployable systems. For example, when an impact with an object is imminent, pre-crash triggered airbags can be deployed to reduce the severity of the impact to the occupant of the vehicle. This is because through deployment of the airbag system prior to impact, the inflated airbag can be better positioned and adequately inflated to provide enhancements in the mechanical interaction between the occupant and the vehicle to provide greater energy absorption, thus reducing the severity of injuries to the vehicle occupant during the ride-down and crush after the impact.
For the pre-crash triggered protection system to operate properly, a robust and reliable sensing system is necessary. Unlike crash sensors which trigger a deployment of a safety system while the vehicle is crushing and decelerating, the sensing system for a pre-crash triggered protection system must anticipate an impact before contact has occurred. This critical “Time Before Collision” is related to the time to deploy the actuator or pyrotechnic device (e.g. 0-200 ms) and the clearance distance between the object and the vehicle (e.g. 100-800 mm). These parameters are particularly critical in side impact conditions. Inadvertent deployment of pyrotechnic safety devices is not only costly but may temporarily disable the vehicle. Moreover, since the deployment of many systems is achieved through a release of energy, deployment at an inappropriate time may result in undesirable effects.
Radar detection systems have been studied and employed for motor vehicles for many years. Radar systems for motor vehicles operate much like their aviation counterparts in that a radio frequency signal, typically in the microwave region, is emitted from an antenna on the vehicle and the reflected-back signal is analyzed to reveal information about the reflecting target. Such systems have been considered for use in collision mitigation by braking systems for motor vehicles, as well as obstacle detection systems for driver convenience functions. Radar sensing systems also have applicability in deploying external airbags. Radar sensors provide a number of valuable inputs, including the ability to detect the range of the closest object with a high degree of accuracy (e.g. 5 cm). They can also provide an output enabling measurement of a closing velocity to a target with high accuracy. The radar cross section of a target and the characteristics of the return signal may also be used as a means of characterizing the target.
Although information obtained from radar systems yield valuable data, exclusive reliance upon a single radar sensor signal for deploying a pyrotechnic device, such as for example an airbag, has certain negative consequences. In particular, in the most simple implementation based on a single sensor signals, a single failure can lead to an inadvertent deployment signal.